feat(notifications): add scheduled email notification system

- NotificationSchedule + NotificationRecipient models with initial migration
- emails.py BUILDERS registry + events_expiring_this_week HTML email builder (IST week bounds)
- send_scheduled_notifications management command (croniter due-check + select_for_update(skip_locked))
- 6 admin API endpoints under /api/v1/notifications/ (types, schedules CRUD, recipients CRUD, send-now)
- date_from/date_to filters on EventListView for dashboard card
- croniter>=2.0.0 added to requirements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

notifications/models.py

@@ -1,4 +1,16 @@
+"""
+Two distinct concerns live in this app:
+
+1. ``Notification`` — consumer-facing in-app inbox entries surfaced on the mobile
+   SPA (/api/notifications/list/). One row per user per alert.
+
+2. ``NotificationSchedule`` + ``NotificationRecipient`` — admin-side recurring
+   email jobs configured from the Command Center Settings tab and dispatched by
+   the ``send_scheduled_notifications`` management command (host cron).
+   Not user-facing; strictly operational.
+"""
 from django.db import models
+
 from accounts.models import User
 
 
@@ -23,3 +35,68 @@ class Notification(models.Model):
 
     def __str__(self):
         return f"{self.notification_type}: {self.title} → {self.user.email}"
+
+
+class NotificationSchedule(models.Model):
+    """One configurable recurring email job.
+
+    New types are added by registering a builder in ``notifications/emails.py``
+    and adding the slug to ``TYPE_CHOICES`` below. Cron expression is evaluated
+    in ``Asia/Kolkata`` by the dispatcher (matches operations team timezone).
+    """
+
+    TYPE_EVENTS_EXPIRING_THIS_WEEK = 'events_expiring_this_week'
+
+    TYPE_CHOICES = [
+        (TYPE_EVENTS_EXPIRING_THIS_WEEK, 'Events Expiring This Week'),
+    ]
+
+    STATUS_SUCCESS = 'success'
+    STATUS_ERROR = 'error'
+
+    name = models.CharField(max_length=200)
+    notification_type = models.CharField(
+        max_length=64, choices=TYPE_CHOICES, db_index=True,
+    )
+    cron_expression = models.CharField(
+        max_length=100, default='0 0 * * 1',
+        help_text='Standard 5-field cron (minute hour dom month dow). '
+                  'Evaluated in Asia/Kolkata.',
+    )
+    is_active = models.BooleanField(default=True, db_index=True)
+    last_run_at = models.DateTimeField(null=True, blank=True)
+    last_status = models.CharField(max_length=20, blank=True, default='')
+    last_error = models.TextField(blank=True, default='')
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ['-created_at']
+        indexes = [models.Index(fields=['is_active', 'notification_type'])]
+
+    def __str__(self):
+        return f'{self.name} ({self.notification_type})'
+
+
+class NotificationRecipient(models.Model):
+    """Free-form recipient — not tied to a User row so external stakeholders
+    (vendors, partners, sponsors) can receive notifications without needing
+    platform accounts."""
+
+    schedule = models.ForeignKey(
+        NotificationSchedule,
+        on_delete=models.CASCADE,
+        related_name='recipients',
+    )
+    email = models.EmailField()
+    display_name = models.CharField(max_length=200, blank=True, default='')
+    is_active = models.BooleanField(default=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        unique_together = [('schedule', 'email')]
+        ordering = ['display_name', 'email']
+
+    def __str__(self):
+        label = self.display_name or self.email
+        return f'{label} ({self.schedule.name})'